Paleoliquefaction in the Bajo Segura Basin (Eastern Betic Cordillera)
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Revistes Catalanes amb Accés Obert ACTA GEOLOGICAHISPANICA, v. 36 (2001), nº 3-4, p. 233-244 Paleoliquefaction in the Bajo Segura basin (eastern Betic Cordillera) Paleolicuefacción en la cuenca del Bajo Segura (Cordillera Bética oriental) P.ALFARO(1 ) , J. DELGADO(1 ) , A. ESTÉVEZ(1 ) and C. LÓPEZ-CASADO(2 ) (1) Departamento de Ciencias de la Tierra y del Medio Ambiente. Universidad de Alicante. Apdo. 99. E-03080. Alicante (Spain). [email protected] / [email protected] / [email protected] (2) Departamento de Física Teórica y del Cosmos. Universidad de Granada. Av. Severo Ochoa s/n. Granada (Spain) ABSTRACT The Bajo Segura basin, in the eastern Betic Cordillera, displays a seismic activity characterized by small-magnitude earthquakes (1.5-4.5 mb), with some occasional moderate to high-magnitude events (> 5.0 mb). These earthquakes are produced by the activity of blind faults without surface ruptures. For this reason, the detection of paleoearthquakes in the geological record is limited to indirect evidence of paleoseismicity, mainly liquefaction features. Moreover, such evidence is abundant in the historical record of the 1829 Torrevieja earthquake, in some of its aftershocks and in the 1919 Jacarilla earthquake. In this study several layers of Holocene seismites previously described in the basin were analyzed, and correlated with various radiometric 1 4C datings. T h i s analysis enabled a recurrence period of approx. 1000 yr to be established for the moderate to high-magnitude earthquakes in the Bajo Segura basin. Keywo rd s : Paleoliquefaction. Paleoseismology. Betic Cordillera. Bajo Segura basin. RESUMEN La cuenca del Bajo Segura, localizada en la Cordillera Bética oriental, tiene una actividad sísmica caracterizada principalmente por terremotos de pequeña magnitud (1.5-4.5 mb), aunque ocasionalmente han ocurrido terremotos de magnitud moderada-alta (> 5 mb). Estos terremotos están producidos por la actividad de fallas que no presentan ruptura en superficie (fallas ciegas). Por este motivo, el reconocimiento de paleoterremotos en el registro geológico se limita a las evidencias indirectas de paleosismicidad entre las que desta- can las estructuras de licuefacción sísmica. Además se tiene constancia de numerosas manifestaciones de licuefacción en el terremoto histórico de Torrevieja de 1829, en alguna de sus réplicas y en el terremoto de Jacarilla de 1919. En este trabajo se han analizado varios niveles de sismitas holocenas descritos previamente en la cuenca, y se han correlacionado con varias dataciones radiométricas de 14 C. 233 Este análisis ha permitido establecer un periodo de recurrencia de terremotos de magnitud moderada-alta en la cuenca del Bajo Segura, de aproximadamente 1000 años. Pal ab r as cla v e : Paleolicuefacción. Paleosismicidad. Cordillera Bética. Cuenca del Bajo Segura. IN T RO D U C T I O N available on the seismic liquefaction structures is s u fficiently detailed. Munson et al. (1995) gave an The Bajo Segura basin, situated in the eastern Betic approximate location for this based on the size of these Cordillera, displays seismic activity characterized by structures. Estimating the magnitude of paleo- small-magnitude earthquakes despite the occurrence of earthquakes, Ambraseys (1988) suggested that a occasional catastrophic events such as the 1829 minimum magnitude of 5 is required to trigger Torrevieja earthquake. Based on the damage caused by liquefaction, and above 5.5, their effects are relatively this earthquake, it has been estimated that the intensity at widespread. Recently, various methods have been used the epicenter reached the degree X (Muñoz et al., 1984). to estimate the magnitude of paleoearthquakes (see Other authors, on the basis of the spatial distribution of discussion in Obermeier, 1996). the damage, have estimated its magnitude as being between 6.3 and 6.9 Ms (Muñoz and Udías, 1991; The Bajo Segura basin contains sediments that are Delgado et al., 1993). Other historical earthquakes, highly susceptible to liquefaction. Seismites (s e n s u reaching an intensity of VIII, occurred at Guardamar del S e i l a c h e r, 1969) have been identified in the upper Segura (1523) and Jacarilla (1919). Given this low- Miocene (Montenat, 1980; Estévez et al., 1994) and moderate seismic activity, the information available for Quaternary deposits (Alfaro et al., 1996, 1999). evaluating the regional seismic hazard is scarce. For this M o r e o v e r, liquefaction occurred during the 1829 reason, there is considerable interest in any study of Torrevieja and 1919 Jacarilla earthquakes. This study paleoseismicity which expands the seismic catalog and analyses the susceptibility of the upper Pleistocene and allows the recurrence period of large earthquakes to be Holocene sediments filling the basin to liquefaction. In established with greater precision. addition, several layers of Holocene seismites, previously described by Alfaro et al. (1996), together In the Bajo Segura basin, the principal active faults with various radiometric 1 4C datings of the Holocene are blind with the result that they do not rupture the sediments (Soria et al., 1999), were used to establish the ground surface. In this region, the only information recurrence period for the moderate to high-magnitude available to date about paleoearthquakes is provided by earthquakes. seismic liquefaction features. Obermeier (1996) made a detailed analysis of the TE C T ONIC SETTING current state of knowledge of the use of coseismic liquefaction features in studies of paleoseismology. Regional seismotectonic studies show that the Bajo These data can provide information about the position Segura basin (Fig. 1) is currently subject to a of the epicenter, the magnitude of the paleoseismic compressive stress field with a maximum horizontal event and the recurrence period of earthquakes of stress, trending NNW-SSE (Alfaro, 1995). This stress moderate-high magnitude. Paleoliquefaction studies field, which is linked to the convergence of the African have identified several paleoearthquakes that occurred and Eurasian plates, is consistent with the recent stress during the Holocene in different regions of the United tensor obtained by the SIGMA project corresponding to States, such as the New Madrid seismic zone (Saucier, the eastern part of the Betic Cordillera (Herraiz et al., 1991; Tuttle and Schweig, 1995) and the South 2000). Carolina coast (Obermeier et al., 1987; Amick et al., 1990; Amick and Gelinas, 1991). Based on dating of Compression has been active since at least the late some paleoearthquakes, recurrence times of moderate Miocene, and has deformed both the basement of the to high-magnitude earthquakes in the New Madrid Internal and External Zones of the Cordillera and the seismic zone have been established (Russ, 1979; Neogene-Quaternary sedimentary cover. A number of S a u c i e r, 1991). In addition, it is possible to determine folds formed in the Neogene-Quaternary sediments. the epicentral region provided that the information Some of these are associated at depth with various blind 234 Figure 1. Geological map of the study area showing location of boreholes and the main outcrops where seismites were observed. Figura 1. Mapa de localización geológica del área de estudio, en el que se han situado los principales afloramientos de sismitas y los sondeos estudiados. reverse faults and strike-slip faults. On the basis of the North, which is characterized by a more marked neotectonic and seismotectonic studies in the Bajo Segura subsidence, and by the La Mata-Torrevieja lagoons to the basin and in adjacent sectors (Montenat, 1977; Bousquet, South. 1979; López Casado et al., 1987; Somoza, 1993; Tab o a d a et al., 1993; Alfaro, 1995), the following faults stand out The Crevillente reverse fault, which runs N070E and as the most active: the Bajo Segura fault, the Crevillente dips northwards, is situated in the northern sector of the fault and the faults running NW-SE, particularly the San basin. The hanging wall is mainly composed of rocks Miguel de Salinas fault (Fig. 1). from the basement of the External Zones of the Betic Cordillera, whereas the footwall comprises sedimentary The ENE-WSW Bajo Segura fault is situated in the rocks from the late Miocene-Quaternary which show a southern part of the Bajo Segura basin. It is a blind progressive unconformity. Several anticlines which reverse fault which dips towards the South and is related, continued to fold during the Quaternary occur along this on the surface, to several asymmetrical anticlines in the fault (Alfaro, 1995). Neogene and Quaternary rocks. These folds include the Guardamar-El Moncayo, Benejúzar and Hurchillo folds In the Bajo Segura basin there are several NW- S E (Montenat, 1977; Taboada et al., 1993; Alfaro, 1995). On running faults, such as the Torrevieja, Guardamar and both sides of the anticlines lie two subsiding sectors San Miguel de Salinas faults. These right-lateral faults represented by the flood plain of the river Segura to the displaced several of the anticlines of the Bajo Segura. 235 SUSCEPTIBILITY OF HOLOCENE AND UPPER liquefaction, noting that it is limited to certain PL E I S T OCENE SEDIMENTS TO LIQUE F ACT I O N environments, whilst absent in others. Considering the data compiled by these authors, the susceptibility to Liquefaction occurred on repeated occasions in the liquefaction of the Segura valley sediments is high for Bajo Segura basin. The historical cases of liquefaction fluvial and littoral environments, and moderate for always took place in the fluvial and coastal sediments alluvial fan sediments. It should also be pointed out that, which lie close to the present-day course of the river. in alluvial fan sediments, the water table is at a greater This is a consequence of the susceptibility of these depth with the result that no cases of liquefaction have sediments to liquefaction. been documented for these sediments.